In display devices such as liquid crystal display devices, to obtain high resolution, reduction in the size of pixels is considered. For example, a plurality of pixels are arranged in matrix. Each pixel comprises a plurality of subpixels corresponding to different colors. Each subpixel comprises a pixel electrode and a thin-film transistor connected to the pixel electrode.
When polycrystalline silicon is used for the semiconductor layer of the thin-film transistor, impurities such as P (phosphorus) or B (boron) are implanted into the semiconductor layer (ion implantation) to form the thin-film transistor.